In recent years, semiconductor laser devices (laser diodes) are in increasing demand in the fields such as optical communication, laser printers, and optical disks. Under these circumstances, active research and development have been conducted with respect to various semiconductor laser devices with particular emphasis on those of GaAS type and InP type. In the field of optical information processing, systems for recording and reproducing information by using 780 nm-band AlGaAs type laser diodes as light sources have been put into practical use. Such systems became widespread for use with recording and reproducing compact disks.
However, recently, the increase in storage capacity of these optical disks has come to be strongly demanded. Along with this, it has come to be required to obtain semiconductor laser devices capable of emitting laser light with shorter wavelengths.
AlGaInP type semiconductor laser devices are capable of allowing laser oscillation to be realized at wavelengths of 630 nm to 690 nm in the red region. In the present specification, (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P (0.ltoreq.x&lt;1) is abbreviated simply as "AlGaInP". At present, of various practical semiconductor laser devices, the AlGaInP type semiconductor laser devices are capable of emitting laser light with the shortest wavelengths, so that they hold great promise as next-generation large capacity light sources for optical information recording, in place of AlGaAs type semiconductor laser devices which have been widely used in the past.
For evaluation of semiconductor laser devices, intensity noise and temperature characteristics are important elements in addition to wavelengths of laser light. In particular, when a semiconductor laser device is used as a light source for reproducing optical disks, small intensity noise is very important. This is because intensity noise induces errors while signals recorded on an optical disk are read. The intensity noise of the semiconductor laser device is caused not only by changes in temperature of the device but also by light partially reflected from the surface of the optical disk to the semiconductor laser device. Thus, semiconductor laser devices, which have small intensity noise even when reflected light is fed back to the devices, are indispensable as light sources for reproducing optical disks.
Conventionally, in the case of using AlGaAs type semiconductor laser devices as low output light sources dedicated for reproducing optical disks, saturable absorbers are intentionally formed on both sides of ridge stripes in the devices so as to reduce noise. The use of such a structure enables a longitudinal mode of laser oscillation to be multiple. When the feedback of laser light to a device, the changes in temperature of the device, etc., are caused while laser oscillation is realized in a single longitudinal mode, a slight change in gain peak allows laser oscillation to start in another longitudinal mode close to the longitudinal mode in which the laser oscillation has already been realized. This causes mode competition between the new longitudinal mode and the original longitudinal mode, resulting in noise. Thus, in the case of multiple longitudinal modes, the changes in intensity of each mode are averaged and the intensity of each mode does not change due to the feed-back of laser light to the device, the changes in temperature of the device, etc. This permits stable low noise characteristics to be obtained.
Japanese Laid-Open Patent Publication No. 63-202083 discloses a semiconductor laser device capable of obtaining stable self-sustained pulsation characteristics. According to this publication, a self-sustained pulsation type laser diode is realized by providing a layer capable of absorbing light generated in an active layer.
Furthermore, Japanese Laid-Open Patent Publication No. 6-260716 discloses that the characteristics of red semiconductor laser devices are improved by rendering a bandgap of an active layer almost equal to that of an absorbing layer. FIG. 1 is a schematic cross-sectional view of a conventional self-sustained pulsation type semiconductor laser device disclosed in Japanese Laid-Open Patent Publication No. 6-260716. Hereinafter, this semiconductor laser device will be described with reference to FIG. 1.
Referring to FIG. 1, a buffer layer 1602 made of n-type GaInP, a cladding layer 1603a made of n-type AlGaInP, a strained quantum well saturable absorbing layer 1605a, a cladding layer 1603b made of n-type AlGaInP, a strained quantum well active layer 1604 made of GaInP, a cladding layer 1603c made of n-type AlGaInP, and a strained quantum well saturable absorbing layer 1605b are successively formed on a substrate 1601 made of n-type GaAs. A cladding layer 1606 and a contact layer 1607 made of p-type GaInP are formed respectively in a ridge shape on the strained quantum well saturable absorbing layer 1605b. Both sides of the cladding layer 1606 and the contact layer 1607 are buried with a current blocking layer 1608 made of n-type GaAs. Furthermore, a cap layer 1609 made of p-type GaAs is formed on the contact layer 1607 and the current blocking layer 1608. A p-type electrode 1610 is formed on the cap layer 1609 and an n-type electrode 1611 is formed on a reverse surface of the substrate 1601.
FIG. 2 shows an energy band of the strained quantum well saturable absorbing layers 1605a and 1605b. In the strained quantum well saturable absorbing layers 1605a and 1605b, barrier layers 1701 made of (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P, and well layers 1702 made of Ga.sub.x In.sub.1-x P (film thickness: 100 .ANG.; strain: +0.5% to 1.0%) are alternately layered. In this example, three well layers 1702 are layered. Herein, the bandgap of the strained quantum well active layer 1604 is prescribed to be almost equal to that of the strained quantum well saturable absorbing layers 1605a and 1605b. In the conventional example, an attempt is made to obtain satisfactory self-sustained pulsation characteristics by using this structure.
Compared with the AlGaAs type semiconductor devices, the AlGaInP type semiconductor devices are not likely to allow self-sustained pulsation to be realized. This is attributable to the big difference in gain characteristics therebetween. FIG. 3 shows the dependency of a gain on a carrier density with respect to GaInP and GaAs, which are mainly used for active layers of the AlGaAs type semiconductor devices and the AlGaInP type semiconductor devices, respectively.
In order to attain self-sustained pulsation, it is required that the rate increase of a gain (i.e., a slope of a gain curve) with respect to a carrier density is large. However, it was found to be relatively difficult in attaining self-sustained pulsation with GaInP because the slope of its gain curve is smaller than that of the gain curve of GaAs.
Furthermore, according to the experimental result of the inventors of the present invention, the following was found: In the case of red semiconductor laser devices (AlGaInP type semiconductor laser devices), because of the gain characteristics it is still difficult to obtain stable self-sustained pulsation merely by rendering the bandgap of an active layer equal to that of a saturable absorbing layer, as in the conventional example.
The present invention has been achieved in view of the above-mentioned points, and its objective is to provide a highly reliable semiconductor laser device having stable self-sustained pulsing characteristics, in particular, by appropriately prescribing the doping degree and thickness of a saturable absorbing layer and a spacer layer forming a semiconductor laser.